Thyroid-stimulating Hormone (TSH) in Pregnancy

Thyroid function test results of healthy pregnant women differ from those of healthy nonpregnant women. During pregnancy there are significant changes in the thyroid gland. The size of the gland increases slightly, production of thyroid hormones increase by about 1.5 times, and subsequently iodine demand increases. T4-binding globulin (TBG) concentrations increase in response to estrogens. Thyrotrophic activity of hCG results in a decrease in serum TSH in the first trimester. During pregnancy, therefore, women may have somewhat lower serum TSH concentrations than before pregnancy. In twin pregnancies the downward shift of TSH can be greater than in singleton pregnancies.

Serum TSH and its reference range gradually rise in the second and third trimesters, but it is noteworthy that the TSH reference interval remains lower than in nonpregnant women. However, the 2017 ATA guidelines have recommended shifting the upper limit from 2.5 to 4.0 based on recent large studies. In a small percentage of women, TSH can be very suppressed (<0.01 mIU/mL) and yet still represent a normal pregnancy. There are slight but significant ethnic differences in serum TSH concentrations. African American and Asian women have TSH values that are on average 0.4 mIU/mL lower than in Caucasian women; these differences persist during pregnancy.¹ This assay has a sensitivity of 0.004 uIU/mL and meets all criteria as a third-generation TSH assay.